This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Solution deposition of semiconducting materials has become increasingly attractive because it offers a number of advantages over vacuum deposition in terms of factors including low capital cost in raw materials and equipment, high-throughput production, and better compatibility with flexible or polymeric substrates. Molecular precursors with desired elements mixed on a molecular scale have been actively studied in the deposition of semiconducting materials, especially metal chalcogenides, for thin film transistors (TFTs), solar cells, thermoelectrics, and phase change memory applications. The molecular-level homogeneity enables the precise control of stoichiometry and the enhanced spatial uniformity in as-deposited films, and in turn an improved device performance. However, the negligible solubility of metal and chalcogen sources in many solvents hampers the development of this solution method. In 2004, the hydrazine-based solution was first used by IBM Watson Research Center to dissolve SnS2, yielding a molecular precursor for the active layer in TFTs. This hydrazine-based precursor was successfully extended to the fabrication of CuIn(S,Se)2, Cu(In,Ga)(S,Se)2, and Cu2ZnSn(S,Se)4 thin films. With a hydrazine-based slurry, a record power conversion efficiency for kesterite Cu2ZnSn(S,Se)4 solar cells was achieved at 12.6%. In order to incorporate the insoluble zinc into the hydrazine-based solution, hydrazinocarboxylic acid was introduced into hydrazine resulting in a solar cell efficiency of 8.08%.
Despite the efficacy of using hydrazine, it is highly explosive, toxic, and carcinogenic, and thus various handling limitations are required during the film preparation. Aiming to replace hydrazine, several trials using other solvents and S or Se sources have been conducted. For example, a thiourea-stabilized precursor was used in the studies of Zeng et al., Hao et al., and Si-Nae Park et al. Unfortunately, the films produced using that precursor are either porous or with a substantial residual fine grain layer which limits the further improvement in the cell performance. In another recent study, copper (I) oxide was dissolved in a mixture of 1-butylamine, carbon disulfide, and thioglycolic acid while zinc oxide and tin oxide were separately dissolved in a mixture of 1-butylamine and carbon disulfide. These three solutions were mixed together to get a CZTS precursor before deposition on the substrates. Although a solar cell efficiency of 6.03% was achieved, the dissolution involves heat treatment and the multi-step precursor preparation is relatively complicated. Furthermore, the solvents used in the above molecular precursors are limited to the deposition of a few metal chalcogenides and thus it is very difficult to extend this precursor method to other metal chalcogenide systems.
There is therefore an unmet need for a novel molecular precursor solution and method for depositing inorganic films and synthesizing inorganic nanoparticles.